Prefill valve for molding machines

ABSTRACT

A prefill valve assembly for a hydraulically actuated molding machine, including a poppet shiftable axially within a valve cylinder between open and closed positions, wherein fluid pressure from the main hydraulic circuit is used to move the poppet to closed position rapidly, thereby reducing the overall cycle time of the machine.

United States Patent Mankowsky et al.

[ Dec. 25, 1973 [54] PREFILL VALVE FOR MOLDING 3,456,297 7/1969Andreasson 425 450 MACHINES I [75] Inventors: Vincent]. lviankowsky,Primary Examiner H. A. y, Jr.

Westchester, Clarence J. Ceroke, A

tt0mey--Johnst0n, Root, 0 Keeffe, Kell, Thompson Homewood, both of Ill.

8L Shurtleff [73] Assignee: U.S. Industries, Inc., New York,

22 Filed: Feb. 2 1971 l 1 57 ABSTRACT [21] Appl. No.: 111,929

A prefill valve assembly for a hydraulically actuated [52] US. Cl425/450, 425/150, 425/DIG. 223 molding machine, including a poppetshiftable axially [51] Int. Cl. B29h 5/24 within a valve cylinderbetween open and closed posi- [58] Field of Search 18/30 LV; 425/150,tions, wherein fluid pressure from the main hydraulic 425/450, DIG. 223circuit is used to move the poppet to closed position rapidly, therebyreducing the overall cycle time of the [56] References Cited machine.

UNITED STATES PATENTS 3,084,512 4/1963 Huelskamp 425/450 UX 8 Claims, 5Drawing Figures L s 59 58 E PATENTED 3.781.160

SHEET 1 [IF 5 INVEA'TORS/ VINCENT J, MANKOWSKY. CLARENCE J. CEROKE amgow kwawygflw M 4 ATT'YS PAIENTEflnms 197a SHEET E OF 3 MEDwwwIQ .POJE

INVENTORS: VINCENT J MANKOWSKY ATT'YS PAIENTEDnsczs 197s SHEET 3 BF 3VINCENT J. MANKOWSKY CLARENCE J. CEROKE BY: ga m1, My

ll ATT'YS PREFILL VALVE FOR MOLDING MACHINES BRIEF SUMMARY OF THEINVENTION The invention relates generally to molding machines, and isdirected more specifically to a novel form of prefill valve assemblydesigned to have extremely rapid closing action, thereby reducing thetotal cycle time of the machine.

In injection molding machines a main ram having a movable platen securedthereto, on which is mounted a mold half, is caused to reciprocate andbe advanced to a mold-closed position and retracted to a mold-openposition. The other mold half is mounted on a stationary platen and themain ram, which is advanced to close the mold halves, reciprocateswithin a main cylinder.

Booster or jack rams are customarily utilized for causing a rapidadvance of the main ram toward its closed position. Such jack ram iscommon in prefill valves, and there is also utilized in combinationtherewith a poppet which is moved to a closed position to shut off thesupply of hydraulic fluid from a reservoir to the main ram cavity withinthe ram cylinder.

In prior devices of this nature pilot hydraulic pressure has been usedeither to open and close the poppet, or to close the poppet when othermeans, such as a spring, is used to normally maintain the poppet in openposition. In other constructions spring means have been used to urge thepoppet to closed position, and illustrations of such prior art devicesmay be seen, for example, in the Norman et al. US. Pat. No. 2,878,648issued Mar. 24, 1959, and in the Huelskamp U.S. Pat. No. 3,084,512issued Apr. 9, 1963.

Closing and opening the poppet with pilot pressure or moving one waywith pilot pressure and the other way with a spring is relatively slowcompared to the device of the present invention. The presentconstruction connects one side of the poppet to the main hydrauliccircuit, and because of the comparatively high volume of hydraulic fluidin the main circuit, the poppet is caused to close more rapidly than theprior art structures, resulting in a decrease of the total cycle time.

As an illustrative example, it may be stated that, in a typicalinstallation where the pilot hydraulic system pumps approximately 12gallons per minute and the main hydraulic system pumps 155 gallons perminute, it takes approximately 3.4 seconds to close the poppet usingpilot pressure, whereas a construction utilizing the pressure from themain hydraulic system to close the poppet requires only about 0.31seconds, thereby decreasing the closing time, or increasing the speedthereof better than times.

In view of the foregoing it is, therefore, a principal object of thepresent invention to provide a prefill valve assembly for a moldingmachine which operates more rapidly than has been possible heretofore,thereby reducing the cycle time of the machine.

Another object of the invention is to provide a prefill valve assemblywherein hydraulic pressure from the main hydraulic system is utilizedfor closing the valve,

thereby causing a more rapid operation and a decrease in the cycle timeof the machine.

Other objects and advantages of the invention will become apparent uponreading the following description taken in conjunction with theaccompanying drawings, in which FIG. 1 is a longitudinal fragmentarysectional view through a clamp assembly of an injection molding machineembodying the present invention, and showing the main ram and theprefill valve in their idle positions;

FIG. 2 is a view similar to FIG. 1 but showing the parts in theiractuated or closed position;

FIG. 3 is a longitudinal sectional view through the clamp assembly andprefill valve on a smaller scale, and showing the parts in their idleposition as in FIG. 1, and including a diagrammatic illustration of themain and pilot hydraulic circuits with the valves thereof in a neutralor idle position;

FIG. 4 is a view similar to FIG. 3 showing the various parts in theiractuated positions as in FIG. 2, and the main and pilot hydrauliccircuits with the valves thereof in the position which causes movementof the main ram and prefill valve to their actuated positions; and

FIG. 5 is a diagrammatic illustration of the ram control valve andcircuitry showing the positions of the valve spools after elapse of theproper curing time which cause retraction of the main ram and return ofthe prefill valve to their idle positions.

DETAILED DESCRIPTION Referring now more particularly to the drawings,the prefill valve of the present invention is indicated generally by thenumeral l. The clamp assembly includes a main cylinder 2, a jack ramsupport 3 spaced therefrom, and between which is positioned a valvecylinder generally indicated at 4. These parts are secured together andthe cylinder held between the main cylinder 2 and jack ram support 3 bymeans of a plurality of bolts 5.

A main ram 6 is located within the main cylinder 2 and is adapted toreciprocate within said cylinder between an advanced mold-closedposition and a retracted mold-open position. As mentioned heretofore,the advanced position of the ram may be seen in FIG. 4 and the retractedposition thereof is shown in FIG. 3.

The outer end of the main ram 6 has secured thereto a movable platen 7on which a mold half (not shown) is adapted to be mounted. A pluralityof guide rods 8 extend through suitable openings in the movable platen 7and guide the reciprocation of the main ram to closed and openpositions. The entire assembly is suitably mounted on a frame generallyindicated at 9.

An elongated jack ram cavity 10 extends axially of the main ram 6 and isadapted to receive one end of a jack ram 11. The outer end 12 of thejack ram is suitably mounted on the jack ram support 3.

An axial passageway 13 extending through the jack ram support 3communicates with an axial bore 14 extending through the jack ram 11.The jack ram is stationary and when hydraulic fluid under pressure fromthe main hydraulic circuit enters the passageway 13 and bore 14, thefluid will be forced out of the jack ram .and against the end of thejack ram cavity 10 with sufficient force to advance the main ram 6 toits moldclosed position as shown in FIG. 4.

The main ram 6 is provided with suitable sealing rings 15 and glands 16to prevent leakage of hydraulic fluid from around the main ram.

It will be noted that the main body of the main ram 6 has a smallerdiameter than that of the main cylinder 2, thereby providing the annularspace 17 between the ram and the cylinder. As will be pointed out morespecifically hereinafter, fluid pressure from the main hydraulic circuitis utilized to retract the main ram by directing it into the annularspace 17. The pressure of the fluid will be exerted against the annularshoulder 18 for this purpose.

The space between the jack ram 1 l and the jack ram cavity is alsosealed against the passage of hydraulic fluid by suitable glands 19.

A poppet 20 surrounds the jack ram at one end thereof and is adapted tobe shifted axially within the valve cylinder 4 between the open positionthereof as shown in FIGS. 1 and 3, and the closed position thereof asshown in FIGS. 2 and 4. A pipe or conduit 21 is connected to a hydraulicfluid reservoir (not shown) at one end thereof and to the prefill valveassembly at the other end thereof so that when the parts are in the idleposition and the poppet 20 is open there will be communication betweenthe hydraulic fluid reservoir and the main ram cavity 22 behind the mainram through the conduit 21 and the annular opening 23 between the end 24of the poppet and its seat 25.

The end 24 of the poppet 20 has an external, annular, angularly disposedseat 26 which is adapted to move into closed position against thesimilarly angled annular seat provided adjacent one end of the valvecylinder 4. This arrangement may be more clearly understood by viewingFIGS. 1 and 2.

The valve cylinder 4 is provided with a main body portion 27. The innersurface thereof is so contoured as to provide an internal annular guide28 for the poppet 20. The inner end of the body 27 adjacent the seat 25is provided with an internal annular rib 29 which has the same diameteras the guiding surface 28. Thus, as will be pointed out more clearlyhereinafter, when the poppet is moved toward the closed position theouter surface thereof will reach the internal annular rib 29 and shutoff communication between the fluid reservoir and the main ram cavity 22before it reaches the completely closed position thereof by the seatingof the mating annular seats 25 and 26. This will be evident from viewingFIGS. 1 and 2.

The opposite end of the poppet 20 is provided with an internal, annular,angularly disposed seat 30 which is adapted to bear against the annularshoulder 31 formed on the body of the jack ram support 3. At one side ofthis shoulder 31 there is an annular recess 32 of a smaller diameterthan the shoulder so that when the poppet is in its open position asshown in FIG. 1 with the annular seat 30 bearing against the shoulder31, a part of the seat 30 will be exposed to the annular recess 32.

One or more radially extending passageways 33 connect the passageway 13in the jack ram support 3 with the annular recess 32. Hydraulic fluidfrom the main circuit, when directed through the passageway 13 and bore14, will also enter the passageway 33 and the annular recess 32.

The outer end of the poppet 20 has an external portion 34 of enlargeddiameter which is provided with a plurality of sealing rings 35. Theouter end 36 of the valve cylinder body 27 has an internal portion 37 ofenlarged diameter extending from the internal guiding surface 28. Thus,the poppet is also guided in its axial movement by the surface portion37.

As indicated hereinabove, rapid closing of the poppet is achieved byhydraulic pressure from the main system. In FIG. 1, it will be notedthat when the poppet is open, a part of the valve seat 30 is exposed tothe annular recess 32. When hydraulic fluid from the main system flowsthrough bore 13 in the jack ram support and through bore 14 in the jackram it will move the main ram to its advanced mold-closed position,after which time hydraulic pressure builds up inside of the jack ram andthrough the port 33. This produces pressure from the main hydraulicsystem against the exposed portion of poppet seat 30, thereby unseatingthe poppet when pilot pressure is relieved to tank.

As will be seen presently, when the operation of the main and pilothydraulic systems are described, the pilot pressure holding the poppetopen will be relieved after the main ram reaches its advanced positionand the poppet will be moved to closed position by main hydraulicpressure. As soon as the poppet is unseated the fluid will spread over alarger area and movement thereof will be slowed. Also a cushioningeffect will occur because one of the ports through which pilot hydraulicfluid flows will be closed by the poppet before the other one.

Furthermore, it is to be noted that when the main ram is fully advancedand the poppet is in closed position, as in FIGS. 2 and 4, both the jackram cavity 10 and the main ram cavity 22 will be in communication withthe same hydraulic fluid supply from the main system. The jack ramcavity 10 is connected to the main fluid supply through bores 13 and 14,whereas the main ram cavity 22 will be connected to the main fluidsupply through bore 13, port 33 and the annular space 38 between theouter end 12 of the jack ram and the inner surface of the poppet. Thislatter connection does not occur, however, until after the seat 30 onthe outer end of the poppet moves past the inner end 39 of the jack ramsupport. The dimensions of the various parts of the valve are such that,as the poppet moves toward closed position the inner end 24 thereof willreach the internal annular rib 29 and shut off communication between thefluid reservoir and the main ram cavity through conduit 21 before theseat 30 on the other end thereof clears the inner end of the jack ramsupport to connect the main ram cavity with the main hydraulic system.

This condition prevails, with the poppet closed and both the main ramcavity and jack ram cavity in communication with the main hydraulicsystem, until the curing cycle is completed, at which time the partsreturn to the idle position and the mold is open in readi ness for thenext cycle.

The hydraulic systems which control the operation of the main ram andprefill valve are illustrated diagrammatically and include generally aprefill control valve 40 and a ram control valve 41. The preflll valveincludes a pilot valve or spool 42 and a main valve or spool 43. The ramcontrol valve includes a pilot valve or spool 44 and a main valve orspool 45.

Pilot hydraulic pressure is provided by a pilot system hydraulic pump(not shown) and is delivered to the pilot valves 42 and 44. The spoolsof these valves are shifted to the various positions thereof by suitablesolenoid valves. The pilot pressure flowing through these pilot valvesact to cause a shifting of the main spools 43 and 45, whereupon mainhydraulic pressure created by a main hydraulic system pump (not shown)is delivered to suitable points for actuation of the various parts.

Referring to FIG. 3, as has been noted heretofore, the main ram andprefill valve are in their idle position. In this condition of thesystems pilot pressure has been delivered through lines 46 and 47,through the ports P and A in the valve 42, and thence through line 48,thereby causing a shifting of the main spool 43 toward the left asviewed in the drawing. In this position of the main spool 43 pilothydraulic pressure is delivered through line 49 and through ports P andA in the main spool 43 through line 50, check valve 51 and lines 52 and53 to the port 54 and into the annular space 55 between the poppet 20and the prefill valve cylinder 27.

This same pilot pressure is also delivered from line 50 through lines 56and 57 to port 58 and into the same annular space 55. This pilothydraulic pressure is exerted against the annular shoulder 59 adjacentthe outer end of the poppet 20, thereby urging the poppet to its openposition as shown in FIGS. 1 and 3. It will be noted that the port 58 isoffset axially from the port 54 leading to the annular space 55 so thatas the poppet is moved toward closed position port 58 will be closedbefore port 54, thereby providing a cushioning effect for the rapidclosing of the poppet.

In this position of the parts and the valves it will also be noted thatany main system hydraulic pressure which had been exerted against theouter end of the poppet when it was in closed position is now relievedby reason of the fact that movement of the poppet to open positionforces hydraulic fluid outwardly through the lines 60, 61, 62 and 63through the ports B and T of the main spool 43 and back to the tank offluid reservoir.

Also in this idle position of the parts the pilot valve 44 and mainspool 45 in the ram control valve 41 will be in their positions as shownin FIG. 3. In this neutral position of the pilot valve 44, pilothydraulic pressure will be delivered through line 64, through ports Pand A and to one end of the main spool 45 through lines 65 and 66. Atthe same time the pilot hydraulic pressure will be delivered to theother end of main spool 45 through ports P and B in the pilot valve 44and lines 67 and 68, thereby maintaining the main spool in its neutralposition. Hydraulic fluid from the main system will flow through line 69and into port P in the spool, then out of port T back to the tankthrough line 70.

Main hydraulic pressure will already have been delivered to the annularspace 17 between the main ram 6 and the main cylinder 2 through port 71from lines 72, 73 and 74, but since in the neutral position of the mainspool 45 ports A and B are closed, the hydraulic fluid will bemaintained in the annular space 17 to keep the main ram 6 in itsretracted mold-opened position.

When the main ram 6 was moved to its retracted position, main hydraulicpressure against the ram through the bores 13 and 14 are relievedvand inthe positions of the valves as shown in FIG. 3 this hydraulic fluid wasdelivered through lines 75, 76, 77, 78 and check valve 79 to line 69which returned it to tank.

When it is desired to close the mold and cycle, the machine solenoidvalve SV8B will be energized, thereby shifting pilot valve 44 toward theleft as shown in FIG. 4. As may be observed, this will direct the pilothydraulic pressure through the ports P and B of the pilot valve 44 fromline 64 and thence through lines 67 and 68 to the main spool 45,shifting it toward the left to the position shown in FIG. 4. In thisposition the main hydraulic pressure is directed from the main supplyand through line 69 through ports P and B of the main spool, and thenthrough lines 80, 81, 78, 77, 76 and 75 where it is delivered throughthe bores 13 and 14 into the jack ram cavity, 10, thereby moving themain ram 6 to its advanced mold-closed position.

It will be evident that simultaneously with the shifting of the mainspool 45 to the position shown in FIG. 4 the hydraulic pressure in theannular space 17 holding the main ram retracted will be relieved byallowing the fluid to be delivered through the lines 72 and 73 andthrough the choke valve 82 to line 74 through ports A and T and back tothe tank.

After the main ram has reached its advanced moldclosed position solenoidvalve SV3 will be energized and will shift the pilot valve 42 to theposition shown in FIG. 4 where pilot pressure is delivered through lines46 and 47 through ports P and B to the other end of the main spool 43,thereby shifting it toward the right. In this position of the spool 43the ports P and B are closed, and pilot pressure which has been holdingthe poppet 20 open will now be relieved by directing the fluid thereinoutwardly through the ports 54 and 58 and their respective lines, andthrough ports A and T in the main spool 43 back to the tank.

During this time, as noted hereinabove, the main hydraulic pressure hasbeen building up inside of the jack ram whereupon fluid pressure againstthe seat 30 of the poppet will unseat the poppet and move it towardclosed position. As described hereinabove, when the poppet is unseatedthe fluid will spread over a larger area and slow down the movementthereof, and its movement will be cushioned by first closing off port 58while port 54 is still open. Also, communication between the tank orfluid reservoir and the main ram cavity through conduit 21 will be shutoff when the inner end of the poppet reaches the inner annular rib 29 inthe valve cylinder 27. Continued movement of the poppet to closedposition, where its seat 26 comes against the annular seat 25, will opencommunication between the main hydraulic system and the main ram cavityas heretofore explained.

After the work cycle is completed an electric cure timer of well knownconstruction deenergizes SV3, thereupon moving the pilot valve 42 backto the position shown in FIG. 3 connecting ports P and A therein andmoving the main spool 43 to connect its ports P and A directing fluidpressure against the poppet shoulder to move it back toward openposition. As heretofore noted, this movement also connects the ports Band T in the main spool 43, thereby relieving the pressure ahead of thepoppet.

Also, at this point SV8B is also deenergized and SV8A is energizedthereupon connecting ports P and A in the pilot valve 44 as shown inFIG. 5. This directs the pilot pressure to shift the main spool 45toward the right connecting the ports P and A therein whereupon mainhydraulic pressure is directed through line 74, check valve 83, lines 73and 72, through the port 71 and into the annular space 17 to retract themain ram. In the position of the valves in FIG. 5 it will be noted alsothat the ports B and T of the main spool are connected for directingfluid back to the tank from the jack ram cavity through lines 76, 77, 81and 80.

When the ram returns to its normal retracted position a limit switch isactuated to deenergize SV8A, thereby returning all spools to the idleposition as shown in FIG. 3.

From the foregoing it will be apparent that a novel arrangement has beenprovided in prefill valves for injection molding machines wherein mainhydraulic fluid pressure is utilized to move the poppet to closedposition, thereby enabling this to take place much more rapidly than hasbeen possible heretofore, and resulting in a considerable decrease inthe required cycle time and consequent increase in production.

Changes may be made in the form, construction and arrangement of partsfrom those disclosed herein without in any way departing from the spiritof the invention or sacrificing any of the attendant advantages thereof,provided, however, that such changes fall within the scope of the claimsappended hereto.

The invention is hereby claimed as follows:

1. In a molding machine having a pilot hydraulic circuit and a mainhydraulic circuit, a main cylinder, a main ram reciprocable thereinbetween advanced, mold-closed, position and retracted, mold-open,position, the volume in said cylinder behind said main ram constitutinga main ram cavity, a jack ram cavity in said main ram, a stationary jackram mounted at the outer end thereof on a jack ram support and the innerend thereof extending into said jack ram cavity in said main ram, ahydraulic fluid supply in communication with said main ram cavity whensaid main ram is retracted, a bore extending through said jack ramadapted to have communication with the main hydraulic circuit, wherebymain hydraulic pressure through said bore will move said main ram towardadvanced position, a prefill valve assembly including a valve cylinderextending between said main cylinder and said jack ram support and apoppet surrounding said jack ram and reciprocable in said valve cylinderbetween a closed position wherein communication between said fluidreservoir and said main ram cavity is shut off, and an open positionwherein communication between said fluid reservoir and said main ramcavity is open, means for urging said poppet toward open position, andmeans for directing hydraulic pressure from said main hydraulic circuitinto said valve cylinder and against one end of said poppet forinitiating and maintaining its movement to closed position,

2. The combination of elements defined in claim 1, wherein said meansfor urging said poppet toward open position is pilot hydraulic pressure,and there are means for relieving said pilot hydraulic pressure whensaid main ram reaches the advanced, mold-closed, position.

3. The combination of elements defined in claim 1, wherein said meansfor urging said poppet toward open position is pilot hydraulic pressure,and switch means operable when said main ram reaches the advanced,mold-closed, position thereof to relieve said pilot hydraulic pressure.

4. The combination of elements defined in claim 1, wherein said main ramcavity and said jack ram cavity are both in communication with the mainhydraulic circuit when said poppet is closed.

5. The combination of elements defined in claim 4, wherein said poppetand said valve cylinder are so related that movement of the poppet toclosed position will close communication between said hydraulic fluidreservoir and said main ram cavity before opening communication betweensaid main ram cavity and the main hydraulic circuit.

6. In a molding machine having a pilot hydraulic circuit and a mainhydraulic circuit, a main cylinder, a main ram reciprocable thereinbetween advanced, mold-closed position, and retracted, mold-openposition, the volume in said cylinder behind said main ram constitutinga main ram cavity, a jack ram cavity in said main ram, a stationary jackram mounted at the outer end thereof on a jack ram support and the innerend thereof extending into said jack ram cavity in said main ram, ahydraulic fluid reservoir in communication with said main ram cavitywhen said main ram is retracted, a bore extending through said jack ramadapted to have communication with the main hydraulic circuit, wherebymain hydraulic pressure through said bore will move said main ram towardadvanced position, a prefill valve assembly including a valve cylinderand a poppet around said jack ram reciprocable within said valvecylinder between open and closed positions, an annular shoulder on saidjack ram support providing a seat for one end of said poppet when in theopen position thereof, an annular seat on said valve cylinder for theother end of said poppet when in the closed position thereof, an annularrecess in said jack ram support adjacent said one end of said poppet, apassageway between said bore and said annular recess, hydraulic pressurefrom the said pilot hydraulic circuit normally urging said poppet towardopen position when said main ram is retracted, and means for relievingthe pilot hydraulic pressure against said poppet when main hydraulicpressure has advanced said main ram to moldclosed position, wherebyhydraulic pressure from the main hydraulic circuit through said bore andsaid passageway against said one end of said poppet will rapidly movesaid poppet to closed position and shut off communication between saidreservoir and said main ram cavity.

7. The combination of elements defined in claim 6, wherein said poppetand said valve cylinder are so arranged that, when said poppet is inclosed position, both said bore and said main ram cavity are incommunication with the said main hydraulic circuit.

8. The combination of elements defined in claim 7, wherein movement ofsaid poppet to closed position will shut off communication between saidmain ram cavity and said hydraulic fluid reservoir before openingcommunication between said main ram cavity and the main hydrauliccircuit.

1. In a molding machine having a pilot hydraulic circuit and a mainhydraulic circuit, a main cylinder, a main ram reciprocable thereinbetween advanced, mold-closed, position and retracted, mold-open,position, the volume in said cylinder behind said main ram constitutinga main ram cavity, a jack ram cavity in said main ram, a stationary jackram mounted at the outer end thereof on a jack ram support and the innerend thereof extending into said jack ram cavity in said main ram, ahydraulic fluid supply in communication with said main ram cavity whensaid main ram is retracted, a bore extending through said jack ramadapted to have communication with the main hydraulic circuit, wherebymain hydraulic pressure through said bore will move said main ram towardadvanced position, a prefill valve assembly including a valve cylinderextending between said main cylinder and said jack ram support and apoppet surrounding said jack ram and reciprocable in said valve cylinderbetween a closed position wherein communication between said fluidreservoir and said main ram cavity is shut off, and an open positionwherein communication between said fluid reservoir and said main ramcavity is open, means for urging said poppet toward open position, andmeans for directing hydraulic pressure from said main hydraulic circuitinto said valve cylinder and against one end of said poppet forinitiating and maintaining its movement to closed position.
 2. Thecombination of elements defined in claim 1, wherein said means forurging said poppet toward open position is pilot hydraulic pressure, andthere are means for relieving said pilot hydraulic pressure when saidmain ram reaches the advanced, mold-closed, position.
 3. The combinationof elements defined in claim 1, wherein said means for urgIng saidpoppet toward open position is pilot hydraulic pressure, and switchmeans operable when said main ram reaches the advanced, mold-closed,position thereof to relieve said pilot hydraulic pressure.
 4. Thecombination of elements defined in claim 1, wherein said main ram cavityand said jack ram cavity are both in communication with the mainhydraulic circuit when said poppet is closed.
 5. The combination ofelements defined in claim 4, wherein said poppet and said valve cylinderare so related that movement of the poppet to closed position will closecommunication between said hydraulic fluid reservoir and said main ramcavity before opening communication between said main ram cavity and themain hydraulic circuit.
 6. In a molding machine having a pilot hydrauliccircuit and a main hydraulic circuit, a main cylinder, a main ramreciprocable therein between advanced, mold-closed position, andretracted, mold-open position, the volume in said cylinder behind saidmain ram constituting a main ram cavity, a jack ram cavity in said mainram, a stationary jack ram mounted at the outer end thereof on a jackram support and the inner end thereof extending into said jack ramcavity in said main ram, a hydraulic fluid reservoir in communicationwith said main ram cavity when said main ram is retracted, a boreextending through said jack ram adapted to have communication with themain hydraulic circuit, whereby main hydraulic pressure through saidbore will move said main ram toward advanced position, a prefill valveassembly including a valve cylinder and a poppet around said jack ramreciprocable within said valve cylinder between open and closedpositions, an annular shoulder on said jack ram support providing a seatfor one end of said poppet when in the open position thereof, an annularseat on said valve cylinder for the other end of said poppet when in theclosed position thereof, an annular recess in said jack ram supportadjacent said one end of said poppet, a passageway between said bore andsaid annular recess, hydraulic pressure from the said pilot hydrauliccircuit normally urging said poppet toward open position when said mainram is retracted, and means for relieving the pilot hydraulic pressureagainst said poppet when main hydraulic pressure has advanced said mainram to mold-closed position, whereby hydraulic pressure from the mainhydraulic circuit through said bore and said passageway against said oneend of said poppet will rapidly move said poppet to closed position andshut off communication between said reservoir and said main ram cavity.7. The combination of elements defined in claim 6, wherein said poppetand said valve cylinder are so arranged that, when said poppet is inclosed position, both said bore and said main ram cavity are incommunication with the said main hydraulic circuit.
 8. The combinationof elements defined in claim 7, wherein movement of said poppet toclosed position will shut off communication between said main ram cavityand said hydraulic fluid reservoir before opening communication betweensaid main ram cavity and the main hydraulic circuit.